The present invention relates to a method for reducing the diameter of a glass rod or tube by drawing when the rod or tube is heated and in a softened and plastically deformable condition. More particularly, the invention relates to a method for reducing the diameter of a fused quartz glass rod or tube as a base material or precursor of optical fibers by drawing the rod or tube to impart a controlled and reduced diameter.
One of the essential requirements in the optical fibers of fused quartz glass in respect of the performance thereof is that the diameter of the fibers must be very accurately controlled while the diameter of optical fibers is strongly influenced by the diameter of the quartz glass rod or tube as a base material or precursor thereof from which the optical fibers are obtained by spinning. Accordingly, many attempts and proposals have been made with an object of obtaining a glass or, in particular, quartz glass rod or tube having an exactly and accurately controlled diameter in the art of optical fibers. Most of these attempts and proposals are made in connection with the step of reducing the diameter of a base rod or tube by drawing while the glass rod or tube is heated and in a softened state capable of plastic deformation including the methods and apparatuses for the automatic control of the drawing velocity. In the conventional process of drawing, the quartz glass rod or tube is held horizontally and drawn in the horizontal direction or, alternatively, the base rod or tube is held vertically as gripped at the upper and lower ends thereof each with a clamp and drawing is performed by the downward movement of the lower clamp with simultaneous downward moving of the upper clamp at a somewhat smaller velocity than the upper clamp. In these drawing methods, the quartz glass rod or tube under drawing is usually rotated around its own axis.
In the first method of horizontal drawing, a drawback of downward bending of the glass rod or tube by gravity sometimes takes place in the heated and softened section thereof to cause a warp of the drawn rod or tube having a reduced diameter, although unevenness in the diameter profile of the drawn rod or tube can be mitigated by rotating the rod or tube under drawing, so that this method is not quite satisfactory as a practical process.
In the second method of downward vertical drawing, the glass rod or tube under drawing little suffers from the problem of warp as a consequence of vertical drawing direction so that the thus drawn rod or tube can have a straightforward configuration with little warp even when the descending glass rod or tube under drawing is not axially rotated. A problem in this case, however, is the sagging of the softened glass material in the plastically deformable condition in the portion under heating by its own weight to cause deformation in the outer profile of the drawn glass rod or tube to cause great difficulties in the accurate control of the diameter of the glass rod or tube after drawing. In particular, the stationary state of drawing can be reached only after a longer time from the start of drawing than in the horizontal drawing. When the velocity of drawing is small to obtain a small reduction of the diameter of the glass rod or tube or the temperature of the section under heating is high, sagging of the softened glass material increases so that the glass rod or tube as drawn may have a bulged portion having a larger diameter than the expected value which should have been determined from the descending velocities of the lower and upper clamps. Such a glass rod or tube having a non-uniform diameter profile is of course unacceptable as a commercial product.